So, I I'm 100% sure Elvis has a swim bladder disease. I double checked online just in case, and he fits everything: floats uncontrollably to the top of the tank, struggles greatly while swimming and swims at an unusual angle, a "kinked" spine and lies around, barely moving except when a mad dash is made to the surface for air.

Now, I know how to manage this as best as I can, my only question is... how in the world did he get it?!
I haven't been overfeeding, he doesn't have a swollen belly at all, he hasn't been injured at all from what I know (when cleaning his tank, I scoop him into a cup, so he doesn't even go near a net) - what could have caused this? Poor Elvis, he looks so awkward and keeps giving me this look of "OH GOD HELP ME I'M UPSIDE DOWN D:"

He's in a 3.5 gallon, temp. is at a constant 78 degrees, last water change was today and I do a full change once a week, and a 25% in the middle of the week when I can. I add a small amount of sea salt when I do full changes (he's a tailbiter, argghhh), and usual water conditioner.

At this point I would lower the water level so that he doesn't have to struggle to breath, if you have a filter turn it off, start making daily water changes 100% with Epsom salt 1tsp/gal for 10 days, you can also cover the top of the tank with plastic veggie wrap to get the air above the water warm and humid. Hold all feedings for the first 3 days then feed half the normal amount every other day.

I like to pre-mix my Epsom salt 1tsp/gal in a clean 1g jug to make dosage easier
The Epsom salt will help reduce edema/swelling in the duct, laxative and it has an antibacterial/fungal effects.

Sometimes they can be fallers as a fry but will recover and be sold only for the symptoms to return, or the duct between the swimbladder and stomach can become inflamed/infected, too cold and dry air can cause swimbladder infection, poor water quality can also cause these symptoms.

At this point I would lower the water level so that he doesn't have to struggle to breath, if you have a filter turn it off, start making daily water changes 100% with Epsom salt 1tsp/gal for 10 days, you can also cover the top of the tank with plastic veggie wrap to get the air above the water warm and humid. Hold all feedings for the first 3 days then feed half the normal amount every other day.

I like to pre-mix my Epsom salt 1tsp/gal in a clean 1g jug to make dosage easier
The Epsom salt will help reduce edema/swelling in the duct, laxative and it has an antibacterial/fungal effects.

Sometimes they can be fallers as a fry but will recover and be sold only for the symptoms to return, or the duct between the swimbladder and stomach can become inflamed/infected, too cold and dry air can cause swimbladder infection, poor water quality can also cause these symptoms.

Thankyou so much :D
I actually have a makeshift lid we made a few months ago, an old portfolio cover with holes poked in it that fit the top of the tank perfectly, haha~